Comments

build_user_type_conversion_1 complains about ambiguity if
LOOKUP_COMPLAIN is set, so we need to avoid setting that flag when we
call the function from reference_binding. It seems to make sense to
mask away all inappropriate lookup flags at the top of
implicit_conversion rather than doing it further down and also in
reference_binding.
Tested x86_64-pc-linux-gnu, applied to trunk.